Student: Stewart Williams
Qualifying Exam Date: Friday, July 9th, 2021
Time: 10:00 a.m.
Zoom Link: https://riceuniversity.zoom.us/j/99053568415?pwd=TEtPNGFZdHRVeUV4d25KQmlrRGRSdz09
1. Preferential flowpaths drive concentration-discharge relationships in the tropics
The relationship between solute concentrations (C) vs. discharge (Q) of a watershed is used to observe coupled hydrochemical processes of earth’s surface, specifically to investigate controls on the net export of solutes due to the chemical weathering of bedrock. In the tropics, shallow preferential flow is commonly cited as a driver of the “flashy” hydrologic response to precipitation events, but this concept has not yet been shown to be relevant to C-Q relationships. To independently determine the activation of shallow subsurface flowpaths, I employ a newly developed technique using isotopic tracer cycles in precipitation and streamflow to determine the “young water fraction,” which describes the fraction of streamflow less than a threshold age of 2-3 months. A 2-year time series of weekly water isotope measurements is presented which spans 6 catchments and 5 precipitation collectors in the Luquillo mountains of eastern Puerto Rico. Young water fractions are calculated for a range of flow regimes and compared with multivariate solute mixing analysis to extend the results across a dataset which spans 30 years of intensive sampling. For all 6 watersheds studied, solute concentrations show a linear relationship with derived young water fractions, demonstrating the importance of preferential flow as a dilute endmember contribution to streamflow. The activation of preferential flowpaths shows a non-linear relationship with discharge, which masks the apparent mixing relationship when only C-Q data is available. The results from this study emphasize the need to apportion streamflow into contributing water masses which follow chemically distinct subsurface flowpaths, and that C-Q relationships are largely driven by the mixing ratio between such flowpaths.
2. Effects of mineral weathering on the mobility of soil organic matter
Soil organic matter (SOM) associated with mineral surfaces has been shown to be highly persistent through time, such that andosols (volcanic soils with high mineral content) have a high capacity for long-term SOM storage. However, the accumulation of SOM in andosols worldwide has been shown to be less than 25% of the flux of CO2 due to volcanism, which shows that soils developed from volcanic materials do not significantly offset the C cycle perturbations associated with volcanic activity (Zehetner 2010). This finding was based on the differences in C density across soil chronosequences, or co-localized sites of different substrate ages, and therefore represents the stationary accumulation of SOM in andosols through time. However, organic matter within soil is inherently mobile, and the total C flux associated with andosols may be underestimated if SOM is mobilized continuously via soil particle erosion or leaching of dissolved organic matter. I propose to investigate SOM mobility across a toposequence (soils along a hillslope representing stable, eroding, and depositional settings) in Iceland. Having formed in an active volcanic region, Icelandic soils are accumulated by persistent tephra deposition over time, periodically interrupted by tephra beds from violent eruptions which have well-constrained ages. A comparison of the difference between tephra-derived soil age and radiocarbon-derived SOM age between the different landforms will aid in identification of SOM which is mobilized across the hillslope or down the soil column. The proposed analysis of the age, amount, and structure of eroded soil C in solid and dissolved phases will yield valuable information on the C flux of OM mobilized from volcanic soils where the effects of chemical weathering are maximized.